How’s the air down there?

All animals need oxygen. Land animals have it easy, with all this air just floating about free for the breathing, but marine animals rely on oxygen that is dissolved in the water. Oxygen can only dissolve into the ocean from the surface, so it’s a limited resource. That’s why there’s natural low-oxygen habitats, like deep in the mud where oxygen can’t penetrate, and unnatural low-oxygen habitats, like in the Gulf of Mexico Dead Zone or in the Chesapeake. The unnatural low-oxygen habitats are caused by sewage and fertilizer pollution – the nutrients cause an algae bloom, the algae dies, falls to the bottom, decays, and the decaying process sucks out all the oxygen. (In the US Northwest, there’s a different process involving naturally low-oxygen waters getting blown onshore by the wind.) This process is called hypoxia.

During a hypoxic event, everything that can’t get out of the way can die. Fish and crabs flee into shallow waters (which are closer to the surface, so they have more oxygen) and there can be massive die-offs of everything from mussels to sea stars. So hypoxia is considered to be a kind of toxin by regulatory agencies such as the EPA. Like all toxins, it’s important to know what dose is harmful. In the case of hypoxia, it’s considered to be less than 2 milligrams of oxygen per liter of water.

However, a recent analysis in PNAS from Raquel Vaquer-Sunyer and Carlos Duarte has found that significant harm occurs way before oxygen sinks to 2 mg/L. They analyzed 872 experiments that examined oxygen tolerance in 206 species of marine bottom-dwelling organisms. Though the median lethal concentration (LC50, when half of the organisms die) was around 2 mg/L, crustaceans were far more sensitive – some species died when oxygen was double that of hypoxia. Fish and crustacean showed physical distress and avoidance behavior at three times the hypoxic limit, with poor overfished cod becoming stressed at an airy 10 mg/L.

The researchers also looked at the amount of time different groups could tolerate 2 mg/L hypoxia before half of them died. (LT50, median lethal time). Once again, fish and crustaceans were most sensitive, with flounder only able to tolerate 23 minutes of hypoxia before expiring. Animals that don’t move very much were more tolerant, with molluscs being able to take over 100 hours of hypoxia. In all cases, larvae were far more sensitive than adults.

So why is this important? The key is in the variance – the little bars sticking out of the box plot. That means that while some species in each group are relatively tolerant of hypoxia, others are so sensitive that they die far before oxygen levels fall to 2 mg/L. Essentially, hypoxia is more “toxic” to ecosystems than we have thought – damage to the ecosystem can occur well before oxygen levels fall to hypoxic levels.

There are more than 400 hypoxic “dead zones” worldwide, and they’re growing. A recent paper in Science found that they cover an area the size of Oregon and are doubling in size every decade. And this only counts the areas that meet ther 2 mg/L definition of hypoxic – Vaquer-Sunyer and Duarte’s work shows that damage could be occuring on an even more massive scale.

To end on a cheery note (because I’m just filled with California sunshine!), the good news about hypoxia is that it has a clear cause and a clear solution. Stop putting fertilizer into the ocean and the problem goes away. The Black Sea began to recover when the collapse of eastern European economies in the 1990s meant that they could no longer afford fertilizer. So there you go – the economic crisis in the US could reduce the size of the Gulf of Mexico Dead Zone! Now, don’t you feel better?

5 Responses to How’s the air down there?

Re ”Beyond Carbon: Scientists Worry About Nitrogen’s Effects” (Sept. 2): It may be possible to employ environmental nitrogen pollution in such a way as to significantly reduce global carbon emissions.

Consider: an estimated one million tons of algae were recently cleared from Chinese coastal waters slated for Olympic sailing events and, while truly impressive in scope, this amount represented only a small fraction of the total Qingdao bloom, which itself, in turn, is only one of many worldwide.

Carbon neutral algal biomass, whether directly harvested or obtained from ”intentional” blooms created at pollution source points, could conceivably supplement on a large scale the burning of coal in solid fuel power plants. Thomas N. Sullivan

I must point you to one success story in the battle to keep fertilizers and other pollutants out of our rivers. I live in Washington County Oregon where we have a county agency called Clean Water Services. A big part of their work is sewage treatment, but they are also the active watchdog over the cleanliness of the Tualatin River Watershed.

The border of the watershed is nearly coterminous with the county’s border which makes administration of their mandate easier. Their web site makes interesting reading. They have done and continue to do experiments with constructed and natural wetlands for the “finishing” of treated waste water. Jackson Bottom Wetlands Preserve is their laboratory and because I do several hours of volunteer work there a month I have been able to observe their work first hand.

I am impressed with all they do. Their staff is knowledgeable and dedicated to maintaining and improving the water quality in our growing valley. As I say, they are a success story and could be a model for other similar governmental agencies.

Peter – I’m afraid that wordpress.com does not allow for direct emailing of posts, so copying the link is the best way.

I’ve heard of fish being able to increase their hemoglobin’s affinity for oxygen (here’s a paper on cichlids), but I’m not sure about multiple hemoglobin types. Here’s one paper on croaker that shows multiple hemoglobin types but not necessarily a clear correlation with hypoxia.